[llvm-dev] LLVM Pass To Remove Dead Code In A Basic Block

[Aaron via llvm-dev]

Hi Dean,

Thanks for your reply.

That's exactly what I am doing, but I was looking for a default
optimization or pass implementation if there was.

I used BasicBlock::splitBasicBlock() but it puts "br" end of original basic
block. I tried to delete the br instruction by using eraseFromParent() but
it didn't work.

I had to rewrite my own splitBasicBlock() by modifying the original one.
Just removed the br instruction creation lines.

>> Just curious, how are you getting return instructions in the middle of a
basic block?
My code generation pass allows multiple return instruction generation since
that simplifies front-end significantly.

Here is the code if anyone would like to use it:

void CodeGenPass::DeleteDeadCode(llvm::BasicBlock * basicBlock)
{
    for (auto it = basicBlock->begin(); it != basicBlock->end(); ++it)
    {
        // Split after first return instruction.
        if (it->getOpcode() == llvm::Instruction::Ret)
        {
            ++it;
            // Split only if there is a following instruction.
            if (it != basicBlock->getInstList().end())
            {
                auto deadCodeBlock = SplitBasicBlock(basicBlock, it);
                deadCodeBlock->eraseFromParent();
            }
            return;
        }
    }
}


llvm::BasicBlock * CodeGenPass::SplitBasicBlock(llvm::BasicBlock *
basicBlock, llvm::BasicBlock::iterator it)
{
    assert(basicBlock->getTerminator() && "Block must have terminator
instruction.");
    assert(it != basicBlock->getInstList().end() && "Can't split block
since there is no following instruction in the basic block.");

    auto newBlock = llvm::BasicBlock::Create(basicBlock->getContext(),
"splitedBlock", basicBlock->getParent(), basicBlock->getNextNode());

    // Move all of the instructions from original block into new block.
    newBlock->getInstList().splice(newBlock->end(),
basicBlock->getInstList(), it, basicBlock->end());

    // Now we must loop through all of the successors of the New block
(which
    // _were_ the successors of the 'this' block), and update any PHI nodes
in
    // successors.  If there were PHI nodes in the successors, then they
need to
    // know that incoming branches will be from New, not from Old.
    //
    for (llvm::succ_iterator I = llvm::succ_begin(newBlock), E =
llvm::succ_end(newBlock); I != E; ++I)
    {
        // Loop over any phi nodes in the basic block, updating the BB
field of
        // incoming values...
        llvm::BasicBlock *Successor = *I;
        for (auto &PN : Successor->phis())
        {
            int Idx = PN.getBasicBlockIndex(basicBlock);
            while (Idx != -1)
            {
                PN.setIncomingBlock((unsigned)Idx, newBlock);
                Idx = PN.getBasicBlockIndex(basicBlock);
            }
        }
    }

    return newBlock;
}


Best,

Aaron


On Thu, May 24, 2018 at 9:22 AM, Dean Michael Berris <dean.berris at gmail.com>
wrote:

>
>
> > On 25 May 2018, at 01:46, Aaron via llvm-dev <llvm-dev at lists.llvm.org>
> wrote:
> >
> > Hi all,
> >
> > LLVM optimization pass gives an error "Terminator found in the middle of
> a basic block!" since basic block IR may have multiple "ret" instructions.
> It seems LLVM does not accept multiple return in a basic block by default.
> >
>
> Yes, if you’re inserting terminators in a basic block at the IR level,
> you’re going to have to split it into two different blocks yourself.
> Essentially you’re encountering a legaliser failure here — it’s an
> invariant of a BasicBlock that there are no terminator instructions in the
> body, and the last instruction should be a terminator.
>
> > Is there a specific optimization or pass that I can enable to remove
> unreachable codes in basic blocks?
> >
>
> If you’re writing your own pass, you can do this yourself — after you
> insert the return instruction, just delete the rest of the instructions in
> the basic block after that instruction.
>
> Just curious, how are you getting return instructions in the middle of a
> basic block?
>
> > Best,
> >
> > Aaron
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> -- Dean
>
>
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